Environmental activists outraged over the Keystone XL oil pipeline are threatening to get involved in primaries against Democrats who support the project.

The political arm of the high-profile environmental group 350.org recently endorsed Rep. Edward Markey (D-Mass.) over pipeline supporter Rep. Stephen Lynch (D-Mass.) in the special election to succeed former Sen. John Kerry (D-Mass.). It was the first primary endorsement ever by the group.

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Hedge fund billionaire and environmentalist Tom Steyer and young activists have also jumped in, going after Lynch over his Keystone backing.

Both 350 Action and an aide to Steyer say involvement in other primaries could be in the offing and that Democrats considering supporting Keystone should learn from Massachusetts.

“We've got some time before 2014, but I'd advise their staff to take a close look at Massachusetts,” said 350 Action spokesman Jamie Henn, referring to pro-Keystone Democrats.

A spokesman for Steyer said he “is very mindful on the approach Mayor Bloomberg has taken on gun reform.”

The New York City mayor is bankrolling a pro-gun-control ad campaign to pressure swing-state senators and, in a closely watched Illinois battle, recently supported the pro-gun-control winner of the Democratic House primary. He's looking at 2014 primary and general election races.

“As Tom believes that climate is an issue of our times, both in terms of the economic impact on our country and what it will mean to our children — with Keystone being the current front-lines of this policy fight — he intends to evaluate races where climate is very much on the ballot in terms of the candidates in a particular race,” said Steyer spokesman Chris Lehane.

Lehane said the Massachusetts race provides a model in terms of criteria, noting a “clear distinction on the issue” in an “important” race, and one that involves “being asked to come in by local people in the state or district.”

Stopping Keystone, which would bring oil from Canadian tar sands projects to Gulf Coast refineries, has become a major priority for many environmentalists.

The latest sign came Friday when 17 Senate Democrats, in a largely symbolic vote, supported a pro-pipeline amendment to the nonbinding fiscal 2014 budget plan.

By jumping into Democratic contests, environmentalists would be taking a page from conservative groups that often get involved in GOP primaries.

But it’s unclear whether the two green groups with the biggest political operations — the Sierra Club and the League of Conservation Voters (LCV) — will consider getting involved in primary campaigns against pro-Keystone Democrats.

A Sierra Club spokesman said “Our plans are still in the works for 2014,” while an LCV spokesman said “we’re not going to advertise our political strategy.”

In the nearer term, Henn said senators who voted in favor of Keystone during the budget battle can expect to hear from opponents of the pipeline.

“Our supporters in Delaware, Florida, Colorado and elsewhere will be bird-dogging their Senators or visiting their offices to make their disappointment known and encouraging them to change their position. These Senators have clearly underestimated how strongly their constituents feel about this issue,” he said.

Sens. Chris Coons (D-Del.), Tom Carper (D-Del.), Michael Bennet (D-Colo.) and Bill Nelson (D-Fla.) were among the Democrats that backed the pro-Keystone amendment, which was offered by Sens. John Hoeven (R-N.D.) and Max Baucus (D-Mont.).

This story was first posted at 5:45 p.m. on March 25 and was updated at 10:29 a.m. on March 26.